COMSOL 6.2 - Semitransparent Surface (Surface-to-Surface Radiation Interface)

This node is a variant of the Opaque Surface (Surface-to-Surface Radiation Interface) node. In addition to specular and diffuse reflection, specular transmission can also be considered on the surface. It is available when the is set to in the Surface-to-Surface Radiation interface settings. The node adds a radiosity shape function for each spectral band to its selection and uses it as surface radiosity.

If no radiation is transmitted through the surface, use the Opaque Surface (Surface-to-Surface Radiation Interface) node instead.

Radiation is assumed to occur on both sides of the surface.

This section has fields and values that are inputs to expressions that define material properties. If such user-defined property groups have been added, the model inputs are included here.

There is one standard model input: the T is used in the expression of the blackbody radiation intensity and when multiple wavelength intervals are used, for the fractional emissive power. The temperature model input is also used to determine the variable that receives the radiative heat source. When the model input does not contain a dependent variable, the radiative heat source is ignored.

The default is . When additional physics interfaces are added to the model, the temperature variables defined by these physics interfaces can also be selected from the list. The option corresponds to the minput.T variable, set to 293.15 K by default) and all temperature variables from the physics interfaces included in the model. To edit the minput.T variable, click the button (

), and in the node under , set a value for the in the section.

Select a boundary system from the list to define the reference vector for the azimuthal angle. The first component of the boundary system selected is the reference vector used for directional surface properties. The default is the boundary system of the model, and the list contains any additional boundary system added under the node.

See Coordinate Systems in the COMSOL Multiphysics Reference Manual for more details.

These settings are the same as for the Diffuse Surface (Surface-to-Surface Radiation Interface) node.

By default, the of the surface radiative properties (emissivity and transmissivity) is set to . The and may be set to expressions depending on other variables like the temperature or the spatial coordinates. Change to or options to define dependencies on the polar and azimuthal angles, θ and φ, through the specification of a , fε and a , fτ. When the option is used, select in the list any function of one argument already defined in the component, under the node. For the option, select among the functions of two arguments. By default, only the function is available in the list, defining fε(θ) = 0, fτ(θ) = 0, fε(θ, φ) = 0, or fτ(θ, φ) = 0.

The other dependencies (temperature T, spatial coordinates x) are handled separately in the and expressions, and used to define the total properties:

If is :

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By default, the ε (dimensionless), ρd (dimensionless), and τ (dimensionless) use values . These are properties of the material surface that depend both on the material itself and the structure of the surface. Make sure that a material is defined at the boundary level (by default materials are defined at the domain level).

Select the check box to set specific values on each side. Select the and to have different material properties on each side. The boundary material specified is used only when , , , , , are .

If is or :

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When is set to , enter a value or expression for the ε. The wavelength may be accessed through the rad.lambda variable. Any expression set for the emissivity is then averaged on each spectral band to obtain a piecewise constant emissivity. If the average value of the emissivity on each band is known, you may use instead the option to avoid the evaluation of the average.

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When is set to , enter a value or expression for the ρd. The wavelength may be accessed through the rad.lambda variable. Any expression set for the reflectivity is then averaged on each spectral band to obtain a piecewise constant reflectivity. If the average value of the reflectivity on each band is known, you may use instead the option to avoid the evaluation of the average.

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When is set to , enter a value or expression for the τ. The wavelength may be accessed through the rad.lambda variable. Any expression set for the transmissivity is then averaged on each spectral band to obtain a piecewise constant transmissivity. If the average value of the transmissivity on each band is known, you may use instead the option to avoid the evaluation of the average.

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When is set to , enter a value for the for each spectral band. Within a spectral band, each value is assumed to be independent of wavelength. By default, the same emissivity is defined on both sides. Select the check box and fill the and columns of the table for a specific definition on each side.

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When is set to , enter a value for the for each spectral band. Within a spectral band, each value is assumed to be independent of wavelength. By default, the same reflectivity is defined on both sides. Select the check box and fill the and columns of the table for a specific definition on each side.

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When is set to , enter a value for the for each spectral band. Within a spectral band, each value is assumed to be independent of wavelength. By default, the same transmissivity is defined on both sides. Select the check box and fill the and columns of the table for a specific definition on each side.

Set the surface emissivity to a number between 0 and 1, where 0 represents a diffuse mirror and 1 is appropriate for a perfect blackbody. The proper value for a physical material lies somewhere in between and can be found from tables or measurements.

Set the surface diffuse reflectivity to a number between 0 and 1, where 1 applies to a perfect diffuse surface and 0 is appropriate when reflection is only specular.

Set the surface transmissivity to a number between 0 and 1, where 0 applies to an opaque surface and 1 is appropriate for a fully transparent surface.

The surface specular reflectivity, ρs, is then defined as

Set a value or expression for the θc, which is the minimum angle of incidence (measured between the ray and the normal to the surface) for transmission to occur. Below this angle, all the radiation is reflected. The pi/2 default value ensures that total reflection does not occur by default.